1. Field of the invention
This invention relates to processing a natural gas, a thermally or catalytically cracked gas, or a refinery off gas to produce a methane-rich product, a nitrogen-rich product, a hydrogen-rich stream, or an olefins-rich product therefrom by solvent extraction. It further relates to adapting the extractive flashing and the extractive stripping versions of the Mehra process for processing of such gas streams by using selected physical solvents.
2. Review of the Prior Art
Many hydrocarbon gases, such as natural gas, are contaminated with one or more inert gases which lower their heat content or otherwise impair their marketability. Such inert gases include nitrogen, helium, and argon. Contamination of natural gas with nitrogen is particularly common. Nitrogen may be a natural component or may be caused by nitrogen injections for reviving oil wells in suitable formations, such as in the central and north Texas areas of the United States.
Such contamination by nitrogen has caused the oil producer to curtail oil production because government regulations prevent him from burning the nitrogen-rich associated gas, and both environmental laws and a desire to preserve valuable resources prohibit him from venting the associated hydrocarbons. The oil producer is thus limited by the choice of technology available to him for properly processing the associated gases from an oil well. The prior art technology, which involves cryogenic principles, cannot economically process the natural gas streams which contain more than 3 mol % nitrogen even after subsidization with the revenue from oil production.
Olefins such as ethylene and propylene are present in thermally or catalytically cracked gas streams or in refinery off gases and are commonly associated with large quantities of hydrogen. These gases generally comprise methane, carbon monoxide, carbon dioxide, acetylene, ethane, methyl acetylene, propadiene, propylene, propane, butadienes, butenes, butanes, C.sub.5 's, C.sub.6 -C.sub.8 non-aromatics, benzene, toluene, xylenes, ethyl benzene, styrene, C.sub.9 -400.degree. F. gasoline, 400+.degree. F. fuel oil, and water.
Numerous processes are known in the solvent absorption art for isolation and recovery of olefins from cracked, refinery, and synthetic gases containing these unsaturated compounds. Some processes utilize specific paraffinic compounds as an absorption oil, and others utilize an aromatic absorption oil as a solvent within an absorber column or an absorber-stripper column having a reboiler. In some instances, these processes additionally isolate a methane-rich stream and/or a hydrogen-rich stream.
A wide variety of gaseous streams are to be found in petroleum refineries. Some streams are integral parts of a specific process, e.g., they are recycled from a fractionating column to a reactor. Such a recycle stream may be an impure hydrogen stream which must be purified before returning to the reactor and/or combining with a make-up hydrogen stream. Other such gaseous streams may be a byproduct of a major refinery process and may be sent to one or more other processes which are nearby and require a hydrogen feed stream. As crudes having higher sulfur content and higher carbon-to-hydrogen ratio continue to be processed and as stricter environmental regulations requiring lower sulfur content are passed, the hydrogen demand is expected to grow. Even though a substantial portion of this increased demand will be met by steam reforming of light hydrocarbons and partial oxidation of heavy hydrocarbons, upgrading of existing off-gas streams is a viable alternative.
For example, the byproduct hydrogen stream from an ethylene cracking plant may have a hydrogen content of 75 mol % and may be initially needed as feed to a hydrodealkylation process requiring 95 mol % hydrogen. Or a change in process conditions at a nearby hydroforming plant may create a demand for 99 mol % hydrogen and consequent purification of a 90% hydrogen byproduct stream, for example, that happens to be available.
There is clearly a need in such circumstances to be able to change selectively from one hydrogen purity to another without having to change equipment specifications.
There are many small to medium size off-gas streams that contain hydrogen and heavier hydrocarbons which are currently being sent to the fuel systems of petroleum refineries. A summary of various hydrogen source streams containing approximate concentrations of hydrogen as published in Oil and Gas Journal, Feb. 6, 1984, p. 111, by Wang et al is shown in Table I. In most of the refinery and petrochemical applications where hydrogen is used as a reactant, the desired makeup hydrogen has a purity of about 95%. In order to prevent the build-up of reaction byproducts, such as methane, a portion of the recycle stream is customarily purged. Even though such a stream is relatively small, its concentration of hydrogen represents a loss which must be offset by additional hydrogen makeup.
TABLE I ______________________________________ Sources of Hydrogen Off-Gas Streams Approximate Hydrogen Industry Source Concentration ______________________________________ Refining HT Purge 25-35 FCC Gas 10-15 Cascade Reject 50-60 Methanol Purge Gas 70-80 Ethylene By-Product H.sub.2 60-90 Cracked Gas 10-35 Coke Oven Product Gas 0-5 LPG Dehydrogenation Product Gas 58 Toluene HDA H.sub.2 Purge 57 Cyclohexane H.sub.2 Purge 42 Carbon Black Product Gas 7 Formaldehyde By-product H.sub.2 18 Ammonia Purge Gas 60 ______________________________________
Several processes have been used and are currently available for upgrading the quality of such off-gas streams. These processes, as described by Wang et al in the Oil and Gas Journal article of Feb. 11, 1984, include cryogenic separation, catalytic purification, pressure swing adsorption, and membrane separation. Selection of a suitable process depends upon many factors, some of which are the hydrogen product purity that is desired, hydrogen recovery levels, available pressure drop, pretreatment requirements, off-gas composition, impact of reaction products remaining in the hydrogen product, and turndown capability of the selected process.
The bulk of the industrial hydrogen manufactured in the United States uses the process of steam reforming of natural gas according to the equation 2CH.sub.4 +3H.sub.2 O.fwdarw.CO+CO.sub.2 +7H.sub.2. Other processes utilize partial oxidation of resids, coal gasification, and water hydrolysis, but when proceeding from natural gas to liquid hydrocarbons and then to solid feed stocks, the processing difficulties and manufacturing costs increase.
The impurities usually found in raw hydrogen are CO.sub.2, CO, O.sub.2, N.sub.2, H.sub.2 O, CH.sub.4, H.sub.2 S, and higher hydrocarbons. These impurities can be removed by shift catalysis, H.sub.2 S and CO.sub.2 removal, PSA process, and nitrogen wash. Upgrading of various refinery waste gases is nearly always more economical than hydrogen production by steam reforming. Composition of the raw gas and the amount of impurities that can be tolerated in the product generally determine the selection of the most suitable process for purification.
U.S. Pat. No. 2,187,631 relates to producing unsaturated C.sub.4 and C.sub.5 hydrocarbons by viscosity breaking a heavy resid and thermally cracking the low-boiling oil fraction thereof in combination with a recycle oil to produce conversion products which are fractionated to isolate a light gas-vapor fraction containing unsaturated and aromatic hydrocarbons desired as final products. This fraction is subjected to a selective absorption operation with an aromatic absorption oil which primarily absorbs the di-olefins and the branched chain mono-olefins of C.sub.4 and C.sub.5 saturated and unsaturated hydrocarbons.
U.S. Pat. No. 2,282,549 relates to polymerizing gaseous olefins to light hydrocarbons of gasoline-like character, with or without catalysis. Solvent scrubbing is used to eliminate ethane, hydrogen, etc. by contacting an olefinic gas mixture containing up to 50% of olefins with a solvent at 100-1000 psi so that hydrogen and C.sub.1 + hydrocarbons remain undissolved and can be separated from the scrubbing oil containing dissolved olefinic hydrocarbons. The solvent may be "condensed and thermally stable aromatic hydrocarbons, such as diphenyl, or polymerization products boiling higher than gasoline and produced in the system itself". The solvent must have a high solvent power for the gaseous olefins and relative low solvent power for methane and hydrogen at 100-1000 psi.
U.S. Pat. No. 2,308,856 relates to a continuous process for extracting olefins from gaseous mixtures by countercurrent contact with a selective solvent for olefins such as isoamylether and other higher aliphatic ethers, butylether, amylether, and similar compounds. The solvent is preferably cooled to a temperature of -20.degree. C. to 25.degree. C., depending upon the solvent and the type of gases to be extracted. The pressure in the absorber may vary from 50 to 300 psi.
U.S. Pat. No. 2,325,379 teaches a process for separating a liquid mixture of components by extractive distillation in the presence of a relatively high boiling selective solvent which may be a polar solvent.
U.S. Pat. No. 2,433,286 is directed to extractive distillation of liquid hydrocarbon mixtures with paraffin hydrocarbons as the extraction solvent in a first extractive distillation to produce olefins plus diolefins in the rich solvent and in a second extractive distillation with unsaturated or aromatic hydrocarbons as the solvent at a higher temperature to produce olefins as the raffinate and diolefins in the rich solvent. Paraffins are distilled from the rich solvent of the first extractive distillation and diolefins are distilled from the rich solvent of the second extractive distillation.
U.S. Pat. No. 2,455,803 describes a process for extractive distillation of a vaporizable organic mixture with a solvent comprising (1) a selective solvent and (2) a mutual solvent for the selective solvent and the mixture. The selective solvent must have high selectivity which is frequently coupled with low solvent power, thereby tending to form two liquid layers within the extractor. The purpose of the mutual solvent is to maintain a single liquid phase. The presence of the solvents in the mixture must cause a greater change in the "escaping tendency" of one component of the mixture relative to that of the other components, "escaping tendency" being defined as the potential of one component to pass from one phase to another. Solvents such as furfural and phenol are named as those having preferential solvent power for aromatic over paraffinic hydrocarbons. Suitable mutual solvents are identified as methyl ketone, cyclohexanone, lactonitrile, morpholine, and aromatic hydrocarbons such as benzene, toluene, cumene, mesitylene, and the like.
U.S. Pat. No. 2,511,206 describes a process for producing commercially valuable ethylene in a derivative of acetylene by pyrolytic decomposition of a hydrocarbon to form a complex gaseous mixture containing ethylene, propylene, and acetylene, then absorbing propylene and acetylene in a polyethylene glycol ether to remove a residual gas containing ethylene, stripping the absorbing medium containing the acetylene and propylene to produce a secondary gas containing propylene and acetylene, and finally processing the secondary gas to produce the desired derivative of acetylene.
U.S. Pat. No. 2,516,507 is relevant for its use of an extractive stripper column for separating a gaseous mixture consisting essentially of C.sub.1, C.sub.2, and C.sub.3 hydrocarbons, including ethylene, by countercurrent absorption in a C.sub.5 -C.sub.7 hydrocarbon as absorbent oil. The process is suitably conducted at 80.degree. F. and a pressure of 300 psia. In the extractive distillation column containing a reboiler, there are successive absorption zones for propane, C.sub.2 -hydrocarbons, and methane. When the gaseous mixture to be separated is a wet hydrocarbon gas feed, e.g., such as a wet gas from an oil well or a refinery off-gas comprising C.sub.1 -C.sub.4 hydrocarbons with N.sub.2 and/or H.sub.2, the rate of feeding the lean oil to the column may be adjusted in relation to the composition of the feed, the nature of the absorption oil, and the temperature and the pressure in the column so that substantially all the C.sub.3 materials are absorbed in the propane absorption zone (primary) while a substantial proportion of the gas at the top of the primary zone is withdrawn as a C.sub.2 -concentrate. The proportion withdrawn is selected so that as remaining gas contacts the oil in the C.sub.2 or secondary absorption zone, substantially all of the C.sub.2 content thereof is absorbed in the oil and is thereby returned to the C.sub.3 absorption zone from which it is stripped by the as yet unabsorbed C.sub.3 therein. Similarly, methane and lighter components are withdrawn from the top of the C.sub.2 (secondary) absorption zone, and the methane and lighter components in the remaining gas, when contacted by the lean oil entering at the top of the column, are stripped of methane, leaving hydrogen and nitrogen to leave the column as the overhead stream.
U.S. Pat. No. 2,573,341 relates to recovering olefinic hydrocarbons from refinery off-gases comprising hydrogen in continuous absorber-stripper columns, using aromatic absorption oil at super-atmospheric pressure. Methane, the lighter hydrocarbons, and hydrogen form the overhead from the first column, and ethylene and heavier fractions are in the fat oil. Successively operated columns separate the olefins.
U.S. Pat. No. 2,588,323 describes an absorption process, for recovering olefins from refinery off-gases, which employs an aromatic absorber oil. The process is very similar to the process of U.S. Pat. No. 2,573,341 except that methanol is added to the overhead of both the ethylene fractionator and the de-ethanizer column and is also fed to one or more of the upper intercoolers of the rectifying-absorber column.
U.S. Pat. No. 2,610,704 relates to contacting refinery gas mixtures, typically comprising hydrogen, methane, ethylene, and ethane, with a polar, preferably water soluble, liquid solvent to depress the volatility of ethylene, relative to hydrogen and methane, in a distillation zone within an extractive distillation column. Temperature and pressure were found to be interrelated. Preferably, temperatures are from 0.degree.-120.degree. F. and pressures from 200-300 psi. The solvent may be an aqueous acetone solution containing 96% acetone and 4% water at a ratio of about 2.5-3.5 of solvent to hydrocarbon at the top of the column. The distillate material in the overhead is typically an admixture of hydrogen and methane, substantially all of the ethylene being in the rich solvent. The rich solvent is flashed at a pressure of about 5 psi to vaporize most of the ethylene. The flashed solvent is then stripped of the remaining ethylene by heating. Finally, the recovered ethylene is washed with water to recover solvent vapors.
U.S. Pat. No. 2,780,580 describes a process for countercurrently treating pyrolysis gas with lean oil, having a boiling range of 100.degree.-400.degree. F. The process utilizes a primary absorber for partial recovery of ethane and a secondary absorber to which pyrolysis gas is fed in the midsection thereof and to which both the bottoms of the primary absorber and fresh lean oil are also fed, producing a fat oil which is sent to a distillation column for removing C.sub.2 and to produce a rich oil which is fed to another distillation column to remove C.sub.3. The lean oil circulation is controlled so that upwards of 75% of the ethylene entering the secondary absorber is recovered with the fresh lean oil while not over 75% of the ethane is recovered by the same lean oil fed to the primary absorber.
U.S. Pat. No. 2,804,488 is relevant for its employment of an absorber-stripper and two absorbents (lean oil and ethane) in the recovery of ethylene from a stream of cracked gas. After compression to 180 psi at 45.degree. F., the lean oil removes C.sub.5 + hydrocarbons from the cooled and compressed gas in an absorption zone, producing an overhead gas stream which is dried, cooled to -148.degree. F., and passed countercurrently within a demethanizing absorber to an ethane stream. The overhead from the absorber is composed of uncondensed gases. The rich absorbent may be stripped of methane by distillation in a methane stripper and then split within an ethylene fractionator into a solvent stream (ethane) and an ethylene/acetylene overhead stream which is split by partial condensation into separate acetylene and ethylene streams.
U.S. Pat. No. 2,849,371 describes a process for separating and recovering low boiling components of a natural gas or of a refinery or synthetic gas which is fed to the midsection of an absorber-stripper column to which the lean absorption oil is fed at the top thereof. This absorbent oil is butane at about 60.degree. F. The off-gas from the absorber-stripper column is fed to a secondary absorber to which debutanized gasoline is fed as the absorbent oil at the top thereof to extract the relatively high boiling hydrocarbons and produce a residue gas. The bottoms material from the absorber-stripper column is fed to a depropanizer, and the overhead therefrom is fed to a de-ethanizer.
An absorption process is disclosed in U.S. Pat. No. 3,213,151 for recovering a recycle stream of 50% hydrogen from a gaseous mixture, comprising hydrogen, methane, and normally liquid hydrocarbons, by absorption with pentanes.
A process is disclosed in U.S. Pat. No. 3,291,849 in which toluene, mixed with other alkyl benzenes, is produced as a lean oil which is used in an absorber to purify a make-up hydrogen stream from a catalytic reformer.
U.S. Pat. No. 3,349,145 teaches an improvement in a process for the catalytic hydrodealkylation of an alkyl aromatic hydrocarbon feed in the presence of an excess of hydrogen. The process comprises withdrawing a hydrogen-rich gas from a source of impure hydrogen, containing 50-90 mol % hydrogen, the remainder being C.sub.1 -C.sub.6 paraffins, and countercurrently scrubbing the gas, which is under a pressure of 200-1000 p.s.i.g. and at a temperature below 200.degree. F., with a liquid absorbent consisting essentially of a mixture of C.sub.9 + aromatic hydrocarbons, thereby absorbing a substantial portion of the paraffins in the absorbent. The aromatic hydrocarbons utilized as the liquid absorbent may comprise, either in pure form or in admixture with other aromatics, xylenes and hgher polyalkyl benzenes such as trimethylbenzenes and tetramethylbenzenes. However, alkyl-substituted mononuclear aromatics, having more than three methyl groups per nucleus or having an alkyl group containing more than three carbon atoms, are less preferred because of their higher hydrogen equivalency. When the crude hydrogen contains C.sub.6, C.sub.7, or C.sub.8 paraffins, a preferred absorbent comprises a C.sub.9 + aromatic hydrocarbon, either in pure form or admixed with other C.sub.9 + aromatics, such as propylbenzene, isopropylbenzene, pseudocumene, and mesitylene.
U.S. Pat. No. 4,479,812 provides a continuous fractionation technique for recovering ethylene from an olefinic feedstock comprising C.sub.3 + higher olefins by contacting the olefinic feedstock countercurrently with a liquid solvent stream comprising C.sub.6 + olefinic gasoline range hydrocarbons for selectively absorbing substantially the entire C.sub.3 + olefin components from the feedstock and then withdrawing an ethylene-rich vapor stream from the absorption tower and further contacting the ethylene-rich stream with a distillate range liquid hydrocarbon stream in a sponge absorber to purify the ethylene stream. The absorption tower is an absorber-stripper column having two intercoolers and a reboiler.
U.S. Pat. No. 4,552,572 relates to purification of raw gases derived from coal by high temperature gasification. Suitable purification solvents must have preferential selectivity for hydrogen sulfide over carbon dioxide. They include methanol, N-methyl pyrrolidone, and dimethyl ether of polyethylene glycol. Commonly, the raw gas intended for synthesis is divided into two parts, one of which is passed through a shift reactor to convert a major portion of its cabon monoxide to hydrogen by the shift reaction: CO+H.sub.2 O-CO.sub.2 +H.sub.2. As the purification treatments remove impurities, including CO.sub.2, the shifted gas, which is rich in hydrogen, and the ushifted gas, which is rich in carbon monoxide, may be blended to produce the ratio of hydrogen to carbon monoxide required for a specific synthesis.
An improved extractive flashing version and an improved extractive stripping version of the Mehra Process are respectively described in U.S. Pat. Nos. 4,623,371 and 4,680,042 for separating C.sub.2 + hydrocarbons from a nitrogen-rich hydrocarbon gas containing from 3 to 75 mol % nitrogen, the remainder being hydrocarbons.
Additional Mehra Process applications for processing nitrogen-rich, hydrogen-rich, and olefin-containing gas streams have been described in an article by Yuv R. Mehra entitled "Using Extraction to Treat Hydrocarbon Gases", Chemical Engineering, Oct. 27, 1986, in a paper presented by Yuv R. Mehra entitled "Mehra Process Flexibility Improves Gas Processing Margins" at the 66th Annual Convention of the Gas Processors Association, Mar. 16-18, 1987 at Denver, Colo., in a paper presented by Yuv R. Mehra at the 1987 National Petroleum Refiners Association's Annual Meeting in San Antonio, Tex., Mar. 29-31, 1987, entitled "Recover and Purify Hydrogen Economically", and in an article published in AIChE's Energy Progress, September 1987, page 150, entitled "New Process Flexibility Improves Gas Processing Margins", by Yuv R. Mehra.